In most solenoids, energization of a main operating coil produces a strong electromagnetic field that drives a main plunger, against the bias of a return spring, from a normal or unactuated position to an actuated position. The direction of movement of the plunger may be inwardly of the solenoid, as it is driven to its actuated position; alternatively, the solenoid may be constructed to have the plunger move outwardly of the solenoid housing when driven to its actuated position. In most conventional solenoids, deenergization of the main coil allows the return spring to drive the plunger back to its normal, unactuated position.
There is another type of solenoid, however, that incorporates an auxiliary release coil as well as the main operating coil used to drive the solenoid plunger. This type of solenoid, known as a latching solenoid, incorporates a mechanism to latch the solenoid plunger in its actuated position when the main coil is deenergized. To restore the plunger to its original position, the auxiliary coil is energized to release the latching mechanism and allow restoration by operation of the main plunger return spring. This release action may conveniently be effected by movement of a small latch release plunger incorporated in or connected to the latch mechanism. Latching solenoids of this kind may be employed in a number of different applications, particularly in diesel engines and turbines for which continued operation is independent of the electrical system. An example of a latching solenoid particularly applicable to use in the control of the fuel pump of a diesel engine is presented in Fuzzell U.S. Pat. No. 4,494,096, issued Jan. 15, 1985.
In many applications, particularly those pertaining to engine control, a latching solenoid is subject to severe use requirements; a diesel truck engine, for example, may be started and stopped repeatedly in each day of use. For such applications, the durability of the latching mechanism in the solenoid is a critical factor. In this regard, the latching arrangement disclosed in the aforementioned Fuzzell patent does not afford the durability desirable in many applications; this is equally true of a number of other previously known solenoid latching mechanisms.
In engine control applications and in others, it may be necessary to provide for manual actuation of the solenoid in the event of a failure in the electrical energizing system. Thus, in the event of such a failure, it may be necessary to actuate the solenoid manually to latched condition in order to start the engine or, conversely, it may be essential to actuate the solenoid manually to its normal unactuated condition in order to shut off the engine. In general, previously known manual actuation arrangements for a latching solenoid have presented appreciable technical difficulties and have been less than fully satisfactory with respect to durability and convenience and effectiveness of manual operation.